This invention is generally directed to a brake system for heavy truck trailers which use an air-braked system, and specifically to an antilock brake system for such a trailer.
In a conventional brake system without antilock brakes, when the driver presses the brake pedal, fluid is forced through conduits to the actual brake mechanism at each wheel. The fluid is pressurized and forces a stationary braking surface against a rotating surface which is attached to the wheel. This action slows the wheel through the effect of friction. The braking surfaces are the shoe linings for drum brakes and the pad linings for disk brakes. The rotating surfaces are, respectively, drums or rotors. The slowing wheel also slows the vehicle because of friction between the tire and the road surface. For hydraulic brakes, as used on all cars and smaller trucks, the working fluid is "brake fluid" which is an actual liquid. For large trucks, the working "fluid" is air, which is of course a mixture of gases.
A component known as the "relay valve" is a significant part of an air brake system on a trailer. This component "relays" a control air pressure signal from the driver's brake pedal in the cab to the brakes. This approach is dictated, fundamentally, by brake response time and economic considerations, and in practice by government regulation and industry standardization. The control line is a small diameter tube running the length of the vehicle combination which becomes pressurized when the driver applies the brakes. A separate source of compressed air also exists. This is the "power" air and a reserve is stored in a tank on the trailer. This power air flows to the brakes to effect the actual braking work. The relay valve isolates the control air signal from the power air. Additional complication is dictated by regulation and failure mode considerations.
A disadvantage with conventional brake systems is that the driver can press too hard for the prevailing conditions, for example when driving on snow or wet asphalt. This causes the wheels to stop rotating and to "lock." The vehicle does slow down, but directional control is lost, and, depending on which wheels lock, and on the road conditions, the vehicle may spin.
An anti-lock brake system (ABS) offers significant safety and control advantages over conventional braking systems and ABS is now in common use on most vehicle types. ABS prevents wheel lock-up and, in effect, control brake torques so that the tire to road interface makes most use of the available friction while at the same time providing a reasonable level of lateral stabilizing force. In an emergency or a panic braking situation, ABS functions by keeping the brake pressure at a level just below that which would cause the wheels to lock-up. The vehicle remains stable during emergency or panic braking--that is, the vehicle does not spin. In addition, the vehicle remains steerable during emergency or panic braking. In effect, ABS utilizes the available friction between the tires and the road while ensuring that a high side force (perpendicular to the direction of travel of the vehicle) is also available, thus providing vehicle stability and steerability. Stopping distances are shorter than with locked wheel stops under most conditions. Finally, the lifetime of the tire is increased when ABS is used.
A conventional ABS includes a wheel speed sensing mechanism, an electronic control module (ECM) and a modulation system which controls the brake actuation fluid (air for heavy trucks) pressure. The modulation system consists of one or more modulators and includes a pneumatic control module (PCM).
The wheel speed sensing mechanism, the ECM and the modulation system are additions to the conventional brake system which, for most vehicle types, remains largely unchanged. The wheel speed sensors determine the speed of the wheels and send this information, in an electrical format, to the ECM. The ECM analyzes the wheel speed information and, if appropriate, electrically signals the modulation system to prevent wheel lock-up. The modulation system responds to these signals and physically reduces the brake pressure to optimum level for the conditions. As noted above, the modulation system controls the brake air pressure. This control becomes active only during hard braking, and/or slick conditions, where the wheels would have stopped rotating (lock-up) in the absence of an anti-lock brake system.
The modulator used in the present invention, like the modulators for existing trailer systems, does not modify the power air pressure directly. Instead the modulator used in the present invention modifies the pressure of the control air signal going to the relay valve. The relay valve relays this controlled pressure signal to the power air which flows to the brake actuation mechanism. Thus, the modulator only has to handle the low air volume associated with the control signal, and not the higher volume of air associated with brake actuation.
With a conventional ABS, most of the components which constitute the PCM are integrated with the relay valve. The integrated unit is referred as the "antilock modulator" or the "relay modulator". The ECM and the PCM act together to reduce pressure to a level which minimizes or prevents wheel lock. In many cases, the ECM is also integrated with the relay modulator, or at least attached to the relay modulator with a bracket, so that the whole assembly may be supplied as a unit. The integrated relay valve/PCM is located as close as reasonably possible to, and approximately equidistant from, the brake chambers of the controlled wheels. The predominant semi-trailer type in the United States has four wheel sets so the anti-lock brake system is located in the suspension area between the two axles. This is not an easily accessible location.
The present invention instead integrates the PCM with the ECM and entirely separates the resulting Antilock Control Module (ACM) from the relay valve. The present invention also incorporates other features to make full us of this architecture as described herein. The system of the present invention can be used with relay valves from any manufacturer and provides the brake system designer with additional flexibility in defining the brake system architecture. Significant advantages are also presented to the service technician when the system of the present invention is used as described herein. Other features and advantages will be understood from a reading of the attached specification in combination with a study of the drawings.